Wavelength-division-multiplexing optical amplifier

ABSTRACT

An optical amplifier amplifies a wavelength-division-multiplexed optical input signal composed of optical signals having different wavelengths such that the output level of each of the optical signal is always optimized and such that the difference between the gains of the amplifier on the optical signals is minimized when the number of optical signals is two. The output level of the amplifier is controlled according to the number of the optical signals, to optimize the output level of each of the optical signals.

This application is a Division of Ser. No. 09/048,462 Mar. 26, 1998 ABNand a Division of Ser. No. 08/508,471 Jul. 28, 1998 Pat. No. 5,264,404.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavelength-division-multiplexing(WDM) optical amplifier, and particularly, to an output control systemand output control circuit for individually controlling the levels ofoptical signals, having different wavelengths, contained in awavelength-division-multiplexed optical output signal.

A WDM optical transmission system multiplexes optical signals havingdifferent wavelengths into a wavelength-division-multiplexed opticalsignal and transmits the same through an optical fiber. This system iscapable of increasing the transmission capacity of each optical fiberdepending on the number of optical signals multiplexed.

The optical amplifier for amplifying such a multiplexed optical signalmust properly set the output level of each of the optical signals and,when the number of wavelengths of the optical signals is two, minimizethe effect of the gains of the amplifier on the optical signals.

2. Description of the Related Art

An optical amplifier employing an erbium (Er)-doped optical fiber isoften used because it effectively amplifies the 1.55-μm-band opticalsignals that are generally used in optical communication. This kind ofoptical amplifier is useful as a post-amplifier for a transmitter, apreamplifier for a receiver, or an in-line amplifier for a repeater, toextend the transmission distance at low cost.

Wavelength division multiplexing (WDM) effectively increasestransmission capacity, and multiplexing 1.55-μm-band optical signals ispreferred. To multiplex and demultiplex optical signals of differentwavelengths, optical devices must be employed. Employing opticaldevices, however, causes a loss of transmission distance. To compensatefor the loss, optical amplifiers are needed. A combination of WDM andoptical amplifiers forms a favorable means to transmit many opticalsignals over a large distance.

Optical amplifiers usually employ a constant output control technique.This technique is adopted not only for amplifying a single-wavelengthoptical signal but also for amplifying a wavelength-division-multiplexedoptical signal composed of a plurality of optical signals havingindividual wavelengths, to provide a constant optical output level.

The wavelength-division-multiplexed optical signal usually consists oftwo optical signals having different wavelengths, one in a 1.535-μm bandand the other in a 1.55-μm band. These bands are favorable becauseerbium-doped optical fibers of the optical amplifier provide high gainson signals in the range of 1.52 μm to 1.54 μm and in the range of 1.54μm to 1.57 μm.

According to the constant output control technique, the total outputlevel of the amplifier is constant, and therefore, the level of each ofthe optical signals is obtained by dividing the total output level bythe number of the optical signals. Accordingly, the levels of theoptical signals may be insufficient when they arrive at a receiver. Ifone of the optical signals is absent due to some reason, the level ofthe absent optical signal is distributed to the other optical signals,to increase the levels thereof. This may cause an error at a receiver.On the other hand, if an optical signal having its own wavelength isadded, it will decrease the levels of the other optical signals. This isa first problem of the conventional WDM optical amplifier.

The gain of the WDM optical amplifier has wavelength dependency. Thewavelength dependency sometimes causes a reception error, which isaggravated by fluctuations in a light source in an optical terminal, ina multiplexer-demultiplexer, or in transmission fibers. Namely, thesefluctuations change the levels of the optical signals contained in awavelength-division-multiplexed optical signal at a receiver. Then, thereceiver cannot detect every optical signal. In particular, any opticalsignal at a low level easily causes a reception error. This is a secondproblem of the conventional WDM optical amplifier.

The WDM optical amplifier of the prior art prepares a monitor signal andchecks the level of the monitor signal to keep the optical output of theamplifier constant. Due to the second problem mentioned above, the gainof the amplifier for the optical output is not always equal to that forthe monitor signal. This results in fluctuations in the optical outputof the amplifier. Preparing the monitor signal involves additionalcosts. The monitor signal will not be a true representative of theoptical output of the amplifier if the system for preparing the monitorsignal fails.

The WDM optical amplifier of the prior art involves a gain tilt. Whencollectively amplifying two optical signals with one excitation beam,the optical amplifier causes a gain tilt to weaken one optical signalrelative to the other depending on the wavelength of the excitationbeam, although the amplifier uniformly amplifies a single opticalsignal.

SUMMARY OF THE INVENTION

To solve these problems in the prior art, an object of the presentinvention is to provide a wavelength-division-multiplexing (WDM) opticalamplifier capable of optimizing the output level of each optical signalhaving an individual wavelength, and minimizing the effect, due to theamplifier, on two optical signals having different wavelengths.

In order to accomplish the object, the present invention provides anoptical amplifier for amplifying a wavelength-division-multiplexedoptical input signal composed of optical signals having differentwavelengths, the level of the optical output of the amplifier beingcontrolled according to the number of the optical signals.

To control the level of the optical output of the amplifier, the powerof an excitation beam is controlled. The optical output of the amplifieris controlled so that each of the optical signals has a given outputlevel. Alternatively, the optical output of the amplifier is controlledaccording to an external reference input that is set according to thenumber of the optical signals. The external reference inputcorresponding to the number of the optical signals may be set through aswitch.

The number of the optical signals is detected, and the optical output ofthe amplifier is controlled accordingly. To detect the number of theoptical signals, the input signal is branched, the branched signal isdemultiplexed through a filter into the optical signals, and the opticalsignals are checked to see if they are present. Alternatively, theoptical signals are modulated with different frequencies and aremultiplexed into the wavelength-division-multiplexed optical inputsignal. The input signal is converted into an electric signal, which isseparated into modulation frequency components. The modulation frequencycomponents are checked to see if they are present, to thereby determinethe number of the optical signals. The modulation frequencies areselected to suppress induced Brillouin scattering in an optical fiberthat transmits the optical signal.

The present invention also provides an optical amplifier for amplifyinga wavelength-division-multiplexed optical input signal composed ofoptical signals having different wavelengths, the level of each of thesignals being detected to control the gain of the amplifier so that theweakest one of the optical signals may have a required output level andeach of the optical signals may secure the required output level ormore.

To detect the level of each of the optical signals, the input signal isdemultiplexed through a filter into the optical signals, the opticalsignals are converted into electric signals, and the levels of theelectric signals are measured to determine the levels of the opticalsignals. Alternatively, the optical signals may be modulated withdifferent frequencies and are multiplexed into thewavelength-division-multiplexed optical input signal. The input signalis converted in the amplifier into an electric signal, which isseparated into modulation frequency components. The levels of themodulation frequency components are measured to determine the level ofeach of the optical signals.

The amplifier is provided with a unit for detecting the presence of eachof the optical signals so that any optical signal that is absent isexcluded from the control of securing a required output level for eachof the optical signals that are present.

The present invention also provides an optical amplifier for amplifyinga wavelength-division-multiplexed optical input signal composed of anoptical signal of 1.535-μm-band wavelength and an optical signal of1.55-μm-band wavelength, by sending the input signal to an erbium-dopedoptical fiber and by simultaneously applying an excitation beam of0.98-μm-band wavelength and an excitation beam of 1.48-μm-bandwavelength to the optical fiber, when the gain of the amplifier on theoptical signal of 1.535-μm-band wavelength is greater than that on theoptical signal of 1.55-μm-band wavelength with respect to the excitationbeam of 0.98-μm-band wavelength, and when the gain of the amplifier onthe optical signal of 1.535-μm-band wavelength is smaller than that onthe optical signal of 1.55-μm-band wavelength with respect to theexcitation beam of 1.48-μm-band wavelength.

The excitation beam of 0.98-μm-band wavelength and the excitation beamof 1.48-μm-band wavelength may be multiplexed and applied to the opticalfiber to amplify the input signal. Alternatively, these excitation beamsmay be applied to different locations of the optical fiber,respectively, to amplify the input signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from thedescription, as set forth below, with reference to the accompanyingdrawings in which:

FIG. 1 is a block diagram showing a system for controlling an opticalamplifier;

FIG. 2 is a block diagram showing a WDM optical amplifier according to aprior art;

FIG. 3 is a block diagram showing a WDM optical amplifier involving twooptical signals having different wavelengths, according to a prior art;

FIG. 4(a) shows the gain characteristic of the amplifier of FIG. 3 withrespect to a 1.48-μm excitation beam;

FIG. 4(b) shows the gain characteristic of the amplifier of FIG. 3 withrespect to a 0.98-μm excitation beam;

FIG. 5 is a block diagram showing a gain measuring system of a WDMoptical amplifier;

FIG. 6(a) shows the gain characteristic of a WDM optical amplifier witha small N2/(N2-N1);

FIG. 6(b) shows the gain characteristic of a WDM optical amplifier witha large N2/(N2-N1);

FIG. 7 shows characteristic curves for explaining a gain tilt of a WDMoptical amplifier;

FIG. 8(a) shows changes in the gain characteristic of a WDM opticalamplifier with weak optical signals or a strong excitation beam;

FIG. 8(b) shows changes in the gain characteristic of a WDM opticalamplifier with strong optical signals or a weak excitation beam;

FIG. 9 shows changes in dG/dλ of a WDM optical amplifier with respect toexcitation beams having different wavelengths;

FIG. 10 is a block diagram explaining a first principle of the presentinvention;

FIG. 11 is a block diagram explaining a second principle of the presentinvention;

FIG. 12 is a block diagram explaining a third principle of the presentinvention;

FIG. 13 is a block diagram explaining a fourth principle of the presentinvention;

FIG. 14 shows the gain characteristic of the fourth principle of thepresent invention;

FIG. 15 is a block diagram showing a first embodiment of the presentinvention;

FIG. 16 is a block diagram showing a second embodiment of the presentinvention;

FIG. 17 is a block diagram showing a third embodiment of the presentinvention;

FIG. 18 is a block diagram showing a fourth embodiment of the presentinvention;

FIG. 19 is a block diagram showing a fifth embodiment of the presentinvention; and

FIG. 20 is a block diagram showing a sixth embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a better understanding of the preferred embodiments of the presentinvention, the problem in the prior art will be explained.

FIG. 1 shows a system for controlling an optical amplifier, according toa prior art, employing the constant output control technique. The systemincludes an erbium-doped optical fiber 1, an excitation laser diode 2for applying an excitation beam to the optical fiber 1, amultiplexer-demultiplexer 3 for combining the excitation beam with anoptical signal and providing an optical output signal, and a drivecircuit 4 for driving the laser diode 2.

An optical input signal is sent to the optical fiber 1 through anisolator 5 that isolates the optical fiber 1 from the input. The opticalfiber 1 amplifies the input signal according to the excitation beam andprovides an amplified optical output signal, which is sent out throughan isolator 6.

A branching device 7 branches a part of the optical output signal. Thebranched part is converted by a photosensor 8 into an electric signalwhose level is detected by a level detector 9. A comparator 10 comparesthe detected level with a reference voltage VREF, and according to thedifference between them, controls the drive circuit 4. The drive circuit4 drives the laser diode 2 accordingly. In this way, the strength of theexcitation beam emitted from the laser diode 2 is controlled to keep theoptical output signal constant.

A branching device 11 is connected to the input end, to branch a part ofthe optical input signal. The branched part is converted by aphotosensor 12 into an electric signal. An absence detector 13 checksthe level of the electric signal, and upon detecting an absence of theinput signal, issues a shutdown signal to stop the drive circuit 4 andturn off the excitation beam of the laser diode 2. If the laser diode 2is not turned off, the control system for keeping the optical output ofthe amplifier constant increases its gain to only amplify noise. If anoptical input signal resumes under this state, the system willexcessively increase the optical output to badly affect a receiver.

FIG. 2 shows a wavelength-division-multiplexing (WDM) optical amplifieraccording to a prior art. The amplifier has an optical fiber 14, asemiconductor laser amplifier 15, a branching device 16 for branching apart of an optical output signal of the laser amplifier 15, a drivecircuit 17 for driving the laser amplifier 15, a demultiplexer 18 fordemultiplexing the branched output signal into an optical signal, aphotosensor 19 for converting the optical signal into an electricsignal, and a gain controller 20 for controlling the gain of the laseramplifier 15.

The input signal to the amplifier is a wavelength-division-multiplexedoptical signal composed of optical signals λ1 and λ2, as well as anoptical monitor signal λm. The branching device 16 separates, some ofthe optical output signal, and the demultiplexer 18 separates themonitor signal λm from the branched output signal. The monitor signal λmis converted by the photosensor 19 into an electric signal. The gaincontroller 20 compares the level of the electric signal with a referencevoltage (not shown), and according to the difference between them,controls the drive circuit 17 to keep the gain of the laser amplifier 15constant.

Like the amplifier of the prior art of FIG. 1 for amplifying asingle-wavelength optical signal, the WDM amplifier of the prior art ofFIG. 2 employs a constant output control technique to always provide anoptical output signal of constant strength.

The wavelength-division-multiplexed optical signal amplified by thisamplifier is composed of, for example, two optical signals havingdifferent wavelengths, e.g., a 1.535-μm-band wavelength and a1.55-μm-band wavelength. The reason why these wavelengths are selectedis because an erbium-doped optical fiber of the amplifier shows highgains on signals whose wavelength ranges from 1.52 μm to 1.54 μm and onsignals whose wavelength ranges from 1.54 μm to 1.57 μm.

FIG. 3 shows a WDM optical amplifier for handling two optical signalshaving different wavelengths, according to a prior art. The samereference marks as those of FIG. 1 represent like parts. The amplifierof FIG. 3 has a multiplexer 21 for multiplexing an optical signal 1 of1.535-μm-band wavelength and an optical signal 2 of 1.55-μm-bandwavelength, and a WDM coupler 22 for multiplexing and demultiplexing awavelength-division-multiplexed optical signal and an excitation beam.

The multiplexer 21 multiplexes the optical signals 1 and 2. Themultiplexed signal is sent to an erbium-doped optical fiber 1 through anisolator 5. An excitation beam from an excitation laser diode 2 is sentto the optical fiber 1 through the WDM coupler 22 to amplify the opticalsignals 1 and 2. The amplified signal is sent outside through anisolator 6.

FIGS. 4(a) and 4(b) show the gain characteristic of the opticalamplifier of FIG. 3, in which FIG. 4(a) shows the gain characteristicwith an excitation beam of 1.48-μm-band wavelength (hereinafter referredto as the 1.48-μm excitation), and FIG. 4(b) shows the gaincharacteristic with an excitation beam of 0.98-μm-band wavelength(hereinafter referred to as the 0.98-excitation). In FIG. 4(a), the1.48-μm excitation achieves a large gain on the optical signal 2 of1.55-μm-band wavelength. In this case, the optical signal 1 of1.535-μm-band wavelength is hardly transmitted. In FIG. 4(b), the0.98-μm excitation achieves a large gain on the optical signal 1 of1.535-μm-band wavelength, and the optical signal 2 of 1.55-μm-bandwavelength is hardly transmitted.

FIG. 5 shows a gain measuring system for a WDM optical amplifier. Thesame reference marks as those of FIG. 3 represent like parts. Thissystem has a spectrum analyzer 23 for measuring the spectraldistribution of an optical output signal.

A multiplexer 21 multiplexes an optical signal 1 (λ=1.56 μm) and anoptical signal 2 (λ=1.53 μm to 1.57 μm). The multiplexed signal is sentto an erbium-doped optical fiber 1 through an isolator 5. An excitationlaser diode 2 emits an excitation beam (λ=1.48 or 0.98 μm), which isapplied to the optical fiber 1 through a WDM coupler 22 to amplify themultiplexed signal. The amplified signal is sent to the spectrumanalyzer 23 through an isolator 6, and the spectrum analyzer 23 measuresthe output levels of the optical signals according to their spectra.

When the amplifier of the prior art of FIG. 1 is employed to amplify awavelength-division-multiplexed optical signal, it provides an opticaloutput signal of constant level. Namely, the level of each opticalsignal contained in the output signal is obtained by dividing the totaloutput level by the number of the optical signals. Accordingly, theoptical signals may have insufficient levels when they arrive at areceiver. If one of the optical signals is absent due to some reason,the level of the absent optical signal is distributed to the otheroptical signals, to increase the levels thereof. This may cause an errorat a receiver. On the other hand, if an optical signal having its ownwavelength is added, it will decrease the levels of the other opticalsignals. This is the problem of the conventional WDM optical amplifier.

The gain of the WDM optical amplifier is wavelength dependent. Thewavelength dependency sometimes causes a reception error, which isaggravated by fluctuations in a light source of an optical terminal,multiplexer-demultiplexers, and transmission fibers. Namely, thesefluctuations change the levels of optical signals contained in awavelength-division-multiplexed optical signal from one another at areceiver. Then, the receiver cannot detect the optical signalsindividually. In particular, any optical signal of low level easilycauses a reception error. This is another problem of the conventionalWDM optical amplifier.

The amplifier of the prior art of FIG. 2 prepares a monitor signal andchecks the level of the monitor signal to keep the optical output of theamplifier constant. Due to the problem mentioned above, the gain of theamplifier on the optical output is not always equal to that on themonitor signal. This results in the optical output of the amplifierfluctuating. Preparing the monitor signal involves additional costs. Themonitor signal will not be a true representative of the optical outputof the amplifier if the system for preparing the monitor signal fails.

FIGS. 6(a) and 6(b) explain the gain characteristic of a WDM opticalamplifier, in which FIG. 6(a) is with a small N2/(N2-N1) and FIG. 6(b)is with a large N2/(N2-N1). Here, N1 is the number of photons in aground state, and N2 is the number of photons in an excited state. Anordinate represents emitted energy N2σe (σe being a cross-sectionalemission area) and absorbed energy N1σa (σa being a cross-sectionalabsorption area), and an abscissa represents the wavelength (μm) of anoptical signal.

The difference between the emitted energy N2σe and the absorbed energyN1σa corresponds to the gain of the amplifier. When N2/(N2-N1) is small,i.e., when the distribution of the ground state is sufficiently smalland the distribution of the excited state is sufficiently large, thegain gl (λ=1.535 μm) of the amplifier around a wavelength of 1.535 μm islarger than the gain gh (λ=1.55 μm) of the amplifier around a wavelengthof 1.55 μm. On the other hand, when N2/(N2-N1) is large, i.e., when thedistribution of the excited state is not sufficiently large, gl (λ=1.535μm) is smaller than gh (λ=1.55 μm).

When an excitation beam of 1.48-μm-band wavelength is employed, thecross-sectional emission area extends nearly to 1.45 μm, and therefore,it is difficult to zero the distribution of the ground state. When theamplifier serves as a post-amplifier for a multiplexed signal containingtwo optical signals, N2/(N2-N1) becomes larger so that gl (λ=1.535 μm)becomes smaller than gh (λ=1.55 μm) to cause a gain tilt.

FIG. 7 shows an example of a gain tilt when optical signals 1 and 2 arecollectively amplified with an excitation beam of 1.48 μm in wavelength.An ordinate represents the output level of the optical signal 2, and anabscissa represents the wavelength of the optical signal 2. A curve "A"is obtained when only the wavelength of the optical signal 2 is changedwithout the optical signal 1. A curve "B" is obtained when thewavelength of the optical signal 2 is changed from 1.53 μm to 1.57 μmwith the wavelength of the optical signal 1 being fixed at 1.56 μm.

As shown in the figure, the amplifier shows a flat amplificationcharacteristic with respect to a single optical signal, while it causesa gain tilt with respect to two optical signals that are collectivelyamplified. In the latter case, the output level of the optical signal 2drops in a short-wavelength region.

When an excitation beam of 0.98 μm in wavelength is employed, it willsufficiently reduce the distribution of the ground state, to provide thegain characteristics of FIGS. 6(a) and 6(b). This is because no emissionoccurs around the excitation wavelength of 0.98 μm, unlike theexcitation beam of 1.48 μm.

FIGS. 8(a) and 8(b) show changes in the gain characteristic of a WDMoptical amplifier with respect to an excitation beam of 0.98 μm inwavelength, in which FIG. 8(a) is with a weak optical signal or a strongexcitation beam, and FIG. 8(b) is with a strong optical signal or a weakexcitation beam.

In FIG. 8(a), the gain of the amplifier for an optical signal of 1.535μm in wavelength is larger than that for an optical signal of 1.557 μmin wavelength when the optical signals are weak or the excitation beamis strong. In FIG. 8(b), the gain of the amplifier on the optical signalof 1.535 μm in wavelength is smaller than that on the optical signal of1.557 μm in wavelength when the optical signals are strong or theexcitation beam is weak.

FIG. 9 shows changes in dG/dλ of a WDM optical amplifier due to changesin the wavelength of an excitation beam. The 1.48-μm excitation involvesa transition probability to a ground state, so that transition from FIG.8(a) to FIG. 8(b) is speedier under the 1.48-μm excitation than underthe 0.98-μm excitation when the strength of an optical signal isincreased.

In FIG. 9, the 0.98-μm excitation keeps the shape of FIG. 8(a) in therange surrounded with dotted lines. On the other hand, the 1.48-μmexcitation takes the shape of FIG. 8(b) that causes a gain tilt.

To solve these problems in the prior art, an object of the presentinvention is to provide a WDM optical amplifier capable of alwaysoptimizing the output level of each of optical signals having differentwavelengths and minimizing a difference in the gain of the amplifierwith respect to two optical signals having different wavelengths.

Now, preferred embodiments of the present invention will be explained indetail.

FIG. 10 shows an optical amplifier according to the first principle ofthe present invention. The same reference marks as those of thepreceding drawings represent like parts. This amplifier includes anoptical output controller 25.

An optical input signal is sent to an erbium-doped optical fiber 1through an isolator 5. The optical fiber 1 amplifies the input signalaccording to an excitation beam applied through a WDM coupler 22. Theamplified signal is sent outside through an isolator 6.

A branching device 7 is arranged at an output end, to branch a part ofthe optical output signal. The branched part is converted by aphotosensor 8 into an electric signal, which is sent to the opticaloutput controller 25. The optical output controller 25 compares theelectric signal with a variable reference voltage, and according to thedifference between them, controls a drive circuit 4, which provides anexcitation laser diode 2 with a drive current. The strength of anexcitation beam emitted from the laser diode 2 is controlledaccordingly, to control the level of the optical output signal.

A branching device 11 arranged at an input end separates a part of theoptical input signal. The separated part is converted by a photosensor12 into an electric signal. The level of the electric signal is checkedby an absence detector (not shown). When the input signal is absent, theabsence detector issues a shutdown signal to stop the drive circuit 4and turn off the excitation beam from the laser diode 2.

The variable reference voltage that determines the output level of theamplifier is varied according to the number of optical signals containedin an optical input signal.

Namely, the output level of the amplifier is controlled such that theoutput level of each optical signal is always constant. When the outputlevel of each of "N" optical signals must be Pout, the total outputlevel of the amplifier is controlled to "Pout×N".

FIG. 11 shows an optical amplifier according to the second principle ofthe present invention. The same reference marks as those of FIG. 10represent like parts. This amplifier has a reference voltage generator27 for generating a reference voltage corresponding to the number ofoptical signals contained in a wavelength-division-multiplexed opticalinput signal.

The reference voltage is sent to an optical output controller 25. Theoptical output controller 25 compares the reference voltage with anelectric signal that is provided by a photosensor 8 and corresponds tothe level of an optical output signal. According to the differencebetween the compared signals, the optical output controller 25 controlsa drive circuit 4, which provides an excitation laser diode 2 with adrive current. In response to the drive current, the strength of anexcitation beam emitted from the laser diode 2 is controlled.Consequently, the level of the optical output signal is controlledaccording to the number of the optical signals.

FIG. 12 shows an optical amplifier according to the third principle ofthe present invention. The same reference marks as those of FIG. 11represent like parts.

A branching device 28 divides a wavelength-division-multiplexed opticalinput signal. A WDM filter 29 demultiplexes the input signal intooptical signals having wavelengths of λ1 to λN. Photosensors 301 to 30Nconvert the optical signals into electric signals, respectively.According to the electric signals, absence detectors 311 to 31N detectthe presence or absence of the optical signals, respectively. Accordingto the presence and absence information, an optical signal counter 32provides the number of the optical signals.

According to the number of the optical signals, a reference voltagegenerator 27 generates a reference voltage, which is sent to an opticaloutput controller 25. The optical output controller 25 controls thestrength of an excitation beam emitted from an excitation laser diode 2accordingly. As a result, the level of an optical output signal of theamplifier is controlled according to the number of the optical signalscontained in the input signal.

FIG. 13 shows an optical amplifier according to the fourth principle ofthe present invention. The same reference marks as those of FIG. 3represent like parts. This amplifier has an excitation laser diode 35for emitting an excitation beam of 1.48-μm-band wavelength, anexcitation laser diode 36 for emitting an excitation beam of0.98-μm-band wavelength, and a multiplexer 37. FIG. 14 shows the gaincharacteristic of the amplifier of FIG. 13.

The multiplexer 37 multiplexes the excitation beam of 1.48-μm-bandwavelength and the excitation beam of 0.98-μm-band wavelength, andapplies the multiplexed beam to an erbium-doped optical fiber 1. Thisamplifier shows an intermediate gain characteristic between the gaincharacteristics of FIGS. 4(a) and 4(b). Accordingly, this amplifier iscapable of minimizing the difference between the gains thereof on anoptical signal 1 having a wavelength of 1.535 μm and an optical signal 2having a wavelength of 1.55 μm.

The functions and operations of the present invention are as follows:

(1) The present invention controls the total level of the optical outputof the amplifier according to the number of optical signals of differentwavelengths contained in a wavelength-division-multiplexed optical inputsignal.

Accordingly, a receiver always receives each of the optical signals at arequired level. Even if one of the optical signals is absent, noreception error will occur because the level of each of the remainingoptical signals is kept at the required level irrespective of theabsence.

(2) The output control of the item (1) may be carried out by controllingthe power of an excitation beam applied to the input signal.

(3) The output control of the item (1) may be carried out by keeping thelevel of each of the optical signals constant.

(4) The output control of the item (1) may be carried out according toan external reference input corresponding to the number of the opticalsignals.

(5) The external reference input of the item (4) may be changed througha switch.

(6) The output control of the item (1) may be carried out according tothe number of the optical signals, which is detected by the amplifieritself.

(7) The detection of the number of the optical signals of the item (6)may be carried out by dividing the input signal, demultiplexing thesignal into the optical signals through a filter, and determining thepresence of each of the optical signals.

(8) The detection of the number of the optical signals of the item (6)may be carried out by modulating the optical signals with differentfrequencies, converting the input signal into an electric signal,separating the frequency-modulated components from one another accordingto the electric signal, and determining the presence of each of thefrequency-modulated components.

(9) The frequencies of the item (8) used for modulating the opticalsignals are selected to suppress induced Brillouin scattering in anoptical fiber for transmitting the optical signal.

(10) The present invention detects the level of each of the opticalsignals contained in the input signal and controls the gain of theamplifier so that the weakest one of the optical signals may have arequired output level, and therefore, each of the optical signals mayhave the required output level or above.

Even if the gain of the amplifier has wavelength dependency, each of theoptical signals will have a required reception level to cause noreception error.

(11) The detection of the level of each of the optical signals of theitem (10) may be carried out by separating the optical signals from oneanother through a filter, converting the separated signals into electricsignals, and detecting the levels of the electric signals.

(12) The detection of the level of each of the optical signals of theitem (10) may be carried out by modulating the optical signals withdifferent frequencies, converting the input signal into an electricsignal, separating the frequency-modulated components from one anotheraccording to the electric signal, and measuring the level of each of theseparated components, to determine the level of each of the opticalsignals.

(13) The detection of the level of each of the optical signals of theitem (10) may be carried out with the use of a unit for determining thepresence of each of the optical signals. Absent ones of the opticalsignals are excluded, and the gain of the amplifier is controlled sothat the weakest one of the optical signals may have a required outputlevel, and therefore, each of the optical signals that are present mayhave the required output level or more.

(14) According to the amplifier of the present invention, an opticalsignal of 1.535-μm-band wavelength and an optical signal of 1.55-μm-bandwavelength are transmitted to an erbium-doped optical fiber. When anexcitation beam of 0.98-μm-band wavelength is applied to the opticalfiber, the gain of the amplifier for the optical signal of 1.535-μm-bandwavelength is larger than that for the optical signal of 1.55-μm-bandwavelength. When an excitation beam of 1.48-μm-band wavelength isapplied to the optical fiber, the gain of the amplifier on the opticalsignal of 1.535-μm-band wavelength is smaller than that for the opticalsignal of 1.55-μm-band wavelength.

In this case, the present invention simultaneously applies theexcitation beams of 0.98-μm-band wavelength and 1.48-μm-band wavelengthto the optical fiber, to thereby equalize the gain of the amplifier onthe optical signal of 1.535-μm-band wavelength with that on the opticalsignal of 1.55-μm-band wavelength.

(15) In the item (14), the excitation beams of 0.98-μm-band wavelengthand 1.48-μm-band wavelength may be multiplexed and applied to theoptical fiber.

(16) In the item (14), the excitation beams of 0.98-μm-band wavelengthand 1.48-μm-band wavelength may be applied to different locations of theoptical fiber, respectively.

FIG. 15 shows a WDM optical amplifier according to the first embodimentof the present invention. The same reference marks as those of FIG. 12represent like parts. This amplifier has a reference voltage generator27. The reference voltage generator 27 has an adder 41, for addingvoltage signals corresponding to the number of optical signals ofdifferent wavelengths contained in a wavelength-division-multiplexedoptical input signal to each other, and a level converter 42 forconverting the voltage signal corresponding to the number of the opticalsignals into a reference voltage. A transistor 43 drives an excitationlaser diode 2.

A branching device 28 divides the input signal. The input signal isseparated by a WDM filter 29 into the optical signals having wavelengthsof λ1 to λN, which are converted by photosensors 301 to 30N intoelectric signals indicating the levels of the optical signals. Absencedetectors 311 to 31N normalize each of the levels of the optical signalsto 1V to indicate presence or 0V to indicate absence. The adder 41 addsthe outputs of the absence detectors 311 to 31N to each other, toproduce a voltage corresponding to the number of the optical signals.

The level converter 42 converts the voltage corresponding to the numberof the optical signals into a reference voltage. A photosensor 8provides a voltage representing the level of an optical output signal.An optical controller 25 compares the voltage provided by thephotosensor 8 with the reference voltage from the level converter 42,and according to the difference between them, controls the transistor43. The transistor 43 provides the laser diode 2 with a drive current.The laser diode 2 emits an excitation beam whose strength is controlledso that the level of the optical output signal may correspond to thenumber of the optical signals.

FIG. 16 shows an optical amplifier according to the second embodiment ofthe present invention. This embodiment determines whether or not each ofoptical signals contained in a wavelength-division-multiplexed opticalsignal is present. An optical terminal has a modulator 461 formodulating an optical signal of wavelength λ1 with a frequency f1, and amodulator 462 for modulating an optical signal of wavelength λ2 with afrequency f2. A WDM multiplexer 47 multiplexes the modulated opticalsignals into a wavelength-division-multiplexed optical signal. Anoptical fiber 48 transmits the optical signal and inputs the same intothe optical amplifier. A branching device 28 of the optical amplifierseparates the input signal. A photosensor 49 converts the signal into anelectric signal. A band-pass filter 501 extracts the signal of frequencyf1 from the electric signal. A band-pass filter 502 extracts the signalof frequency f2 from the electric signal. A level decision unit 511determines the level of the signal of frequency f1 and providesinformation indicating the presence or absence of the optical signal ofwavelength λ1. A level decision unit 512 determines the level of thesignal of frequency f2 and provides information indicating the presenceor absence of the optical signal of wavelength λ2.

The optical terminal modulates each optical signal with a frequency(phase) to identify the wavelength of the optical signal. Namely, theoptical terminal modulates an optical signal having a wavelength of λ1with a frequency of f1, and an optical signal having a wavelength of λ2with a frequency of f2. These modulated signals are multiplexed by theWDM multiplexer 47 into a wavelength-division-multiplexed opticalsignal.

The amplifier converts a branched part of the optical input signal intoan electric signal, which is passed through the band-pass filters toextract the optical signals modulated with the frequencies f1 and f2,respectively. The levels of these signals are checked to provideinformation about the presence or absence of the optical signals.Generally, the second embodiment employing the electric filters todetect optical signals is less expensive than the first embodiment thatemploys the WDM filter to separate optical signals.

An optical fiber causes induced Brillouin scattering when the spectralpurity of an optical signal transmitted through the optical fiber ishigh and when the level of the signal is higher than a threshold. Theinduced Brillouin scattering hinders the transmission of the opticalsignal. The induced Brillouin scattering is suppressible by modulatingthe transmission signal with a phase or frequency that expands thespectral band width. This technique increases the level of thetransmission signal. Accordingly, the second embodiment is effective toprevent induced Brillouin scattering.

The modulation frequencies must be sufficiently high to suppress inducedBrillouin scattering. If the frequencies are too high, however, theeffect of suppressing the induced Brillouin scattering deteriorates.This is because, when a modulator modulates the amplitude of a currentto drive a laser diode, to modulate the frequency of a beam emitted fromthe laser diode, the extremely high frequency deteriorates theefficiency between the amplitude modulation and the frequencymodulation. This results in narrowing a frequency modulation width,thereby deteriorating the effect of suppressing the induced Brillouinscattering. Accordingly, modulation frequencies employed by the secondembodiment must properly be selected to suppress the induced Brillouinscattering.

FIG. 17 shows a WDM optical amplifier according to the third embodimentof the present invention. This amplifier is employed as apost-amplifier. The same reference marks as those of FIGS. 12 and 16represent like parts. For the sake of simplicity, awavelength-division-multiplexed optical signal handled by this amplifierconsists of two optical signals having individual wavelengths.

A branching device 7 separates a part of an optical output signal. A WDMfilters 55 demultiplexes the signal into optical signals havingwavelengths λ1 and λ2. Photosensors 561 and 562 convert the opticalsignals into electric signals, respectively. Level detectors 571 and 572detect the levels of the electric signals, respectively.

A branching device 28 separates an optical input signal. The signal isseparated by a WDM filter 29 into optical signals having wavelengths λ1and λ2, respectively. The optical signals are converted by photosensors301 and 302 into electric signals, respectively. According to the levelsof the electric signals, absence detectors 311 and 312 determine thepresence or absence of the optical signals.

Comparators 58 and 59 compare the levels provided by the level detectors571 and 572 with reference levels VREF1 and VREF2, respectively. Thecomparators 58 and 59 provide outputs to logic diodes 62 and 63 tocontrol a drive circuit 4. The drive circuit 4 drives an excitationlaser diode 2. Accordingly, the strength of an excitation beam emittedfrom the laser diode 2 is subjected to feedback control according theweakest one of the optical signals. As a result, the difference betweenthe levels of the optical signals is minimized.

If any one of the absence detectors 311 and 312 detects the absence ofthe corresponding optical signal and provides an output of 0 V, theoutput level of the corresponding comparator drops due to acorresponding one of logic diodes 60 and 61. Then, according to feedbackcontrol corresponding to the optical signal that is present, the drivecircuit 4 drives the laser diode 2. Accordingly, an erbium-doped opticalfiber 1 is never over excited.

As mentioned above, the embodiment of FIG. 17 solves the second problemof the prior art. Namely, this embodiment compensates for a drop in thelevel of a signal due to the wavelength dependency of the gain of theamplifier.

FIG. 18 shows an optical amplifier according to the fourth embodiment ofthe present invention. The same reference marks as those of FIG. 13represent like parts.

An optical signal 1 having a wavelength of 1.535 μm and an opticalsignal 2 having a wavelength of 1.557 μm form awavelength-division-multiplexed optical input signal. The input signalis passed through an isolator 5 and is sent to an erbium-doped opticalfiber 1. An excitation laser diode 35 emits an excitation beam having awavelength of 1.48 μm, and an excitation laser diode 36 emits anexcitation beam having a wavelength of 0.98 μm. A multiplexer 37multiplexes these excitation beams. The multiplexed excitation beam ismade incident to the optical fiber 1 through a WDM coupler 22.

The optical fiber 1 amplifies the input signal accordingly. Theamplified signal is transmitted through an isolator 6.

Since the optical fiber 1 is excited by the excitation beams ofwavelengths of 1.48 μm and 0.98 μm, the amplifier provides a flat gaincharacteristic as shown in FIG. 14 when amplifying an input signalcomposed of optical signals of wavelengths of 1.535 μm and 1.557 μm.

FIG. 19 shows an optical amplifier according to the fifth embodiment ofthe present invention. The same reference marks as those of FIG. 18represent like parts. This embodiment has a WDM coupler 65.

An excitation laser diode 35 emits an excitation beam having awavelength of 1.48 μm, which is made incident to an erbium-doped opticalfiber 1 through a WDM coupler 22. An excitation laser diode 36 emits anexcitation beam having a wavelength of 0.98 μm, which is made incidentto the optical fiber 1 through the WDM coupler 65.

Consequently, an optical input signal to the optical fiber 1 is excitedby the excitation beams of wavelengths of 1.48 μm and 0.98 μm. Thisamplifier provides a flat gain characteristic as shown in FIG. 14 whenamplifying a wavelength-division-multiplexed optical signal composed ofoptical signals having wavelengths of 1.535 μm and 1.557 μm.

FIG. 20 shows an optical amplifier according to the sixth embodiment ofthe present invention. The same reference marks as those of FIG. 18represent like parts.

A branching device 67 separates an optical output signal. The signal isseparated by a WDM filter 68 into optical signals having wavelengths of1.535 μm and 1.557 μm. Photosensors 69 and 70 convert the opticalsignals into electric signals, respectively. A comparator 71 comparesthe levels of the electric signals with each other. According to thecomparison result, a controller 72 increases the strength of anexcitation beam having a wavelength of 1.48 μm emitted from anexcitation laser diode 35 or an excitation beam having a wavelength of0.98 μm emitted from an excitation laser diode 36.

More precisely, if the level of the optical signal of 1.535 μm inwavelength is higher than the level of the other optical signal, thecontroller 72 controls the laser diode 35 to increase the strength ofthe excitation beam of 1.48 μm in wavelength. If the level of theoptical signal of 1.557 μm in wavelength is higher than the level of theother optical signal, the controller 72 controls the laser diode 36 toincrease the strength of the excitation beam of 0.98 μm in wavelength.

This technique minimizes the difference between the levels of theoptical signals.

As explained above, a WDM optical amplifier according to the presentinvention controls the power of an excitation beam emitted from anexcitation laser diode according to the number of optical signalscontained in a wavelength-division-multiplexed optical input signal,thereby controlling the output level of the amplifier. Accordingly, eachoptical output signal of the amplifier of the present invention securesa required level at a receiver, to cause no reception error.

The optical amplifier according to the present invention employs anexcitation beam of 1.48-μm-band wavelength and an excitation beam of a0.98-μm-band wavelength to minimize the wavelength dependency of thegain of the amplifier on optical signals of 1.535-μm- and 1.55-μm-bandwavelengths, to thereby minimize the difference between the levels ofthe optical signals at a receiver and prevent a reception error.

What is claimed is:
 1. A method for amplifying awavelength-division-multiplexed optical signal composed of an opticalsignal of 1.535-μm-band wavelength and an optical signal composed of anoptical signal of 1.55-μm-band wavelength, comprising the stepsof:inputting the wavelength-division-multiplexed optical signal to anerbium-doped optical fiber; amplifying said inputwavelength-division-multiplexed optical signal by providing excitationbeams of 0.98-μm-band wavelength and 1.48-μm-band wavelength into theerbium-doped optical fiber; monitoring each component level output fromsaid optical fiber of the optical signal of 1.535-μm-band wavelength andthe optical signal of 1.55-μm-band wavelength of the amplifiedwavelength-division-multiplexed optical signal; comparing the levels oftwo components of the optical signal of 1.535-μm-band wavelength and theoptical signal of 1.55-μm-band wavelength; controlling the excitationbeams of 0.98-μm-band wavelength and 1.48-μm-band wavelength byincreasing either one of said excitation beams to equalize the twocomponent levels relative to each other by increasing the lower level ofone component to the higher level of the other component.
 2. The opticalamplifier according to claim 1, wherein the excitation beam of0.98-μm-band wavelength and the excitation beam of 1.48-μm-bandwavelength are multiplexed and applied to the optical fiber to excitethe input signal.
 3. The optical amplifier according to claim 1, whereinthe excitation beam of 0.98-μm-band wavelength and the excitation beamof 1.48-μm-band wavelength are applied to different locations of theoptical fiber, respectively, to excite the input signal.